Process for producing high density nuclear fuel particles



United States Patent Ofifice 3,168,470 Patented Feb. 2, 1965 The presentinvention relates to a process for making material useful as a fuelelement in nuclear reactors generally and more in particular to powerreactors of a solid fuel type.

Heretofore, reactors have usually had metallic fuel elements. Certaindisadvantages are inherent in fuel elements of this type and oxide fuelelements have long been sought to replace the metallic elements.

In the preparation of oxide nuclear fuels composed of thorium oxide orthorium oxide combined with uraniurn oxide, it has been known that asatisfactory method of preparing this material is the decomposition of athorium or uranium metal compound to give a metal oxide. Unfortunately,however, the most easily sintered power is so fine that it ispyrophoric. This makes handling and procesing most difiicult. Thismaterial has heretofore been calcined and then formed into pellets,either alone or mixed with other active oxides.

It is an object of the present invention to produce nuclear fuels ofthorium oxide or thorium oxide combined with uranium oxide which is ofmaximum density.

It is an additional object of this invention to provide a process forproducing high density metal oxide powder of the composition describedwhich is suitable for use in nuclear fuel elements.

A still further object of this invention is to produce an oxide fuelelement composed of thorium oxide or thorium oxide combined with uraniumoxide which has maximum density and maximum structural stability.

These and other objects will be apparent to one skilled in the art fromthe following specification, which is limited only by the claimsappended thereto.

Compounds of thorium, either alone or combined with compounds ofuranium, can be processed to form oxide pellets at least 95 percentdense. Generally, compounds such as the sulfates, nitrates, oxalates,carbonates, or hydroxides of'the metals can be used, since they can bethermally decomposed at comparatively low temperatures in a selectedatmosphere. This list is not intended as limiting but merely toillustrate some compounds which can be used. The pellets have highstructural strength, which produces a fuel element for atomic reactors,that has all of the desired properties of an oxide element, and therebyavoids the difficulties encountered with metallic elements. The compoundshould have a particle size greater than 0.1 micron, and it has beenfound that a particle size of 0.1 to 0.5 micron has produced acceptablepellets having a 95 percent density, and that 0.5 micron particles givea higher density pellet with less chance of cracking.

Briefly, the process comprises compacting a thermally decomposablecompound of thorium, either alone or combined with a thermallydecomposable compound of uranium, into a green compact, heating this toa decomposition temperature and then continuing heating to a sinteringtemperature.

COMPACTING Compacting ammonium diuranate under a pressure of not lessthan about /2 ton per square inch will give a compact of sulficientgreen strength to allow firing without the compact breaking frominternal stress, provided the rate at which the green ware is heated tothe decomposition temperature is properly controlled.

Varying amounts of water may be added to assist the compaction process,the amount of water added depending somewhat upon the manner in whichcompaction is effected. In ordinary pressing, for example, as betweenopposed dies, amounts of water from 0 to 10 percent are useful, whilecompacting processes such as extrusion may require as much as 25 to 50weight percent Water.

DECOMPOSING Decomposition of the compound(s) takes place during heatingstarting at room temperature and is completed by 700 C. and the compactmust be heated to this temperature. The rate of heating is critical anddepends on the initial compacting pressure. If the original compactingpressure is /2 ton per square inch, then the green compact may be raisedto the temperature range in which decomposition takes place at a muchfaster rate than can a similar compact which has been pressed at 10 tonsper square inch. The compact that has been prepared at /2 ton pressureper square inch may be heated as fast as about 100 C. per hour up to 700C. to effect decomposition of the diuranate. A compact pressed at 5 tonsper square inch, however, should be raised to 700 C. in increments of 25C. per hour. During the decomposition phase, that is, during the heatingof the compacts to 700 C., the atmosphere within the furnace ispreferably wet hydrogen, which provides additional moisture, insuringthat the process will operate as desired. After the compact has beenbrought to 700 C. and main tained at this temperature for a sufficienttime to complete decomposition, the temperature is raised to thesintering temperature.

When it is desired to produce oxide particles of at least percentdensity rather than integral compacts, the

compound(s) is/ are compacted at higher pressures, for

example, 5 to 10 tons per square inch or greater, and heated to about700 C. at rates in excess of those at which physical integrity of thecompact can be maintained. Generally, a compact produced by pressing atabout 10 tons per square inch will break into cubes inch to /8 inch insize when heated at a rate of at least C. per hour. The heating rate canbe varied somewhat, depending upon the amount of compacting pressureused, the higher pressures permitting slower heating rates and lowerpresures requiring faster heating rates. Having fragmented the compactby rapid heating through the decomposition range, densification of theindividual particles is achieved by heating them to temperatures on theorder of the sintering previously mentioned for integral compacts. Sinceeach particle of the material produced by this means is at least 95percent dense, the material can be used to form long lengths of fuelelements by procedures such as swaging or into more intricate shapes byslip casting. For example, a suitable container can be filled withparticulate metal oxide and then swaged, drawn or extruded to compactthe metal oxide and reduce the cross-sectional area of the compositewhile increasing the length thereof.

SIN TERIN G The compact is raised to the sintering temperature afterdecomposition. The rate of increasing the temperature is not criticaland is commonly 50 to 250 C. per hour. The final sintering temperatureis an inverse factor of the initial compacting pressure. That is, thehigher the initial compacting pressure, the lower the final sinteringtemperature to attain maximum density may be.

A compact produced by pressing at /2 ton per square inch, heated to 700C. as described above, which temperature was achieved at a rate of 100C. per hour in wet hydrogen, must then be raised to a sinteringtemperature of about 1700 C., where it is held in a wet hydrogenatmosphere for about one hour, and then cooled in dry hydrogen at thefurnace cooling rate. This gives a compact of at least 95 percentdensity. This maximum sintering temperature is critical and a compactprepared under a pressure of /2 ton per square inch, heated to only 1500C., does not achieve the necessary density, regardless of the length oftime it is held at this temperature, and 1700 C. as shown above isnecessary.

Example 1 Thorium hydroxide of a particle size between 0.1 and 0.5micron with l percent water is compacted under a pressure of /2 ton persquare inch and then placed in a furnace. The furnace is maintainedunder an atmosphere of wet hydrogen, which is obtained by bubblinghydrogen through water at room temperature (approximately 20 C.).Hydrogen having a dew point of 00 C. would also be acceptable, buthydrogen of a somewhat higher dew point is preferred to insurereproductivity of results. The furnace is then heated at a rate of 100C. per hour up to 700 C. to effect decomposition of the hydroxide. Afterdecomposition, the compact is sintered by raising the temperature atabout 100 C. per hour to 1700 C., maintaining an atmosphere of Wethydrogen, and held at this temperature for about one hour. Then dryhydrogen is introduced into the furnace and the furnace is allowed tocool. This compact will have a density of at least 95 percent.

Example 2 Thorium hydroxide containing 10 percent water is compactedunder a pressure of tons per square inch and is placed in a furnaceunder an atmosphere of wet hydrogen, as in Example 1, and thetemperature is raised 25 C. per hour to 700 C. to decompose thehydroxide. After decomposition, the compact is sintered by raising thetemperature at approximately 100 C. per hour to 1500 C. and held for 8hours in an atmosphere of Wet hydrogen. The furnace is then allowed tocool at the rate of 200 C. to 300 C. per hour in an atmosphere of dryhydrogen. This compact has a 95 percent density.

Example 3 A mixture of 80 percent thorium hydroxide and 20 percenturanium hydroxide containing percent water is prepared of materialhaving the required particle size and compacted under a pressure of 5tons per square inch can be processed in the same manner as the materialof Example 2 and obtain a compact at least 95 percent dense. Similarly,mixtures of compounds of thorium and uranium can be prepared whichdiffer compositionally from that illustrated and obtain compacts ofdesired density and strength.

It has been found that when the sintering temperature of 1500" C. isused, the compact should be held at this temperature for about 4 to 8hours, but when a higher sintering temperature is used, the compact canbe sintered for much shorter times. Sintering temperatures approaching2000 C. might require only a few minutes, while temperatures in therange of 1700 to 1800 C. probably require about one hour.

The sintering time or temperature may be materially reduced if thecompact is pressed at a higher pressure and processed in the mannerdescribed.

While in the above examples, compacting under pressure is required, thepresent invention also contemplates that the material need not bepressed in a die but could be consolidated or agglomerated into desiredform by 1. slip casting or extrusion, or in the case of powderedmaterial by swaging.

The oxides so formed are heated at a rate not less than 100 C. per hourup to 700 C., where the decomposition of the metal compound has beencompleted. It is then heated to a sintering temperature of 1500 C. to1700 C. in the presence of wet hydrogen. After the sintering temperaturehas been reached, dry hydrogen is introduced and the furnace allowed tocool.

The above process produces a compact consisting of oxides of thorium,either alone or combined with oxides of uranium, without additionaloxygen that has at least percent of a maximum theoretical density and istherefore useful as a fuel element.

The use of dry hydrogen after slntering is essential to obtain a compactdevoid of additional oxygen which would render it subject to radiationdamage.

While the best forms of the preferred embodiments of the invention havebeen described as required by law, it will be apparent to those skilledin the art that it may be applied to other systems of producing fuelelements, and changes may be made in the form to suit special conditionsWithin the scope of this invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A process for producing particles at least 95 percent dense for useas an oxide nuclear fuel composed of a metal oxide selected from thegroup consisting of thorium oxide and thorium oxide-uranium oxidemixtures, said process comprising compacting a thermally decomposablecompound of the metal to be used in the oxide fuel element under apressure sufiicient to form a green compact, heating the green compactin a reducing atmosphere to a temperature reducing the metal compound,and performing the heating at a rate causing the green compact tofragmentize due to internally induced stresses.

2. A process for producing particles at least 95 percent dense for useas an oxide nuclear fuel composed of a metal oxide selected from thegroup consisting of thorium oxide and thorium oxide-uranium oxidemixtures, said process comprising compacting the metal compound at apressure of not less than about 10 tons per square inch to form a greencompact, heating the green compact in a reducing atmosphere to atemperature reducing the metal compound, and performing the heating at arate of not less than about C. per hour.

References Cited by the Examiner UNITED STATES PATENTS 2,868,707 1/59Alter et al 252301.1 2,906,598 9/59 Googin 2314.5 2,950,238 8/60Nicholson 17667 2,953,430 9/60 Leaders et al. 23--14.5 2,967,141 1/61Piclclesimer et al 176-69 3,114,689 12/63 Cope 252-301.1

OTHER REFERENCES Nuclear Science Abstract No. 1807, November 1953.

Harrington: Chemical 62 Engineering Progress, vol. 54, No. 3, March1958, pp. 68 and 69.

2d Geneva Conference on Atomic Energy, vol. 6, September 1958, pp. 592,593, 602, 603.

2d Geneva Conference on Atomic Energy, vol. 28, September 1958, p. 218.

CARL D. QUARFGRTH, Primary Examiner.

OSCAR R. VERTIZ, ROGER L. CAMPBELL,

Examiners.

1. A PROCESS FOR PRODUCING PARTICLES AT LEAST 95 PERCENT DENSE FOR USEAS AN OXIDE NUCLEAR FUEL COMPOSED OF A METAL OXIDE SELECTED FROM THEGROUP CONSISTING OF THORIUM OXIDE AND THORIUM OXIDE-URANIUM OXIDEMIXTURES, SAID PROCESS COMPRISING COMPACTING A THERMALLY DECOMPOSABLECOMPOUND OF THE METAL TO BE USED IN THE OXIDE FUEL ELEMENT UNDER APRESSURE SUFFICIENT TO FORM A GREEN COMPACT, HEATING THE GREEN COMPACTIN A REDUCING ATMOSPHERE TO A TEMPERATURE REDUCING THE METAL COMPOUND,AND PERFORMING THE HEATING AT A RATE CAUSING THE GREEN COMPACT TOFRAGMENTIZE DUE TO INTERNALLY INDUCED STRESSES.